In car auto transmissions (ATs) or continuously variable transmissions (CVTs), resin sealing rings are customarily used for sealing an annular gap between a shaft and a housing that rotate relative to each other, in order to maintain a hydraulic pressure. Sealing rings of this type are sometimes fitted in annular grooves provided on the outer circumference of the shaft. When the resin sealing ring is of an endless type without any joints, then in order to fit such a sealing ring into an annular groove, the sealing ring is moved along the shaft to the position of the annular groove while being enlarged radially, after which it may be necessary to correct the form of the sealing ring by reducing its radial size. Because of such poor mountability of the endless sealing ring into an annular groove, sealing rings with joints have been used widely in recent years. While various shapes of joints have been proposed for effectively minimizing leakage of the sealed fluid, there is still a possibility that a small amount of fluid would leak regardless of the shape of a joint.
Meanwhile, for the sealing ring that is mounted into an annular groove of a shaft, it has been known to provide a recess in a side face of the sealing ring as a technique for reducing sliding resistance between the side face of the sealing ring and a side face of the annular groove (see PTL 1). In order to effectively reduce the sliding resistance, it is desirable that a surface receiving sliding contact of the side face of the sealing ring have a small surface roughness. Since it is generally difficult to reduce the surface roughness of a side face of an annular groove, a sealing ring such as that mentioned above has sometimes been used in an annular groove with a side face whose surface roughness was not sufficiently small. In such a case, the sliding resistance of the sealing ring could not be reduced effectively.